1. Field of the Invention
The present invention relates generally to repeatered optical transmission systems, and more specifically to a technique for establishing loopback circuits in the event of a failure in a fiber optic transmission medium between adjacent repeater stations.
2. Description of the Related Art
Each repeater station of conventional repeatered fiber optic transmission systems is provided with eastward and westward repeaters. In the eastward repeater, an optical signal arriving at an incoming eastward fiber optic transmission line is converted by a first electrical-to-optical signal transducer to an electrical signal, passed through an electrical switching circuit to a first optical-to-electrical signal transducer where it is converted to an optical signal and forwarded onto an outgoing eastward fiber optic transmission line. Likewise, in the westward repeater an optical signal arriving at an incoming westward fiber optic transmission line is converted by a second electrical-to-optical signal transducer to an electrical signal, passed through the electrical switching circuit to a second optical-to-electrical signal transducer where it is converted to an optical signal and forwarded onto an outgoing westward fiber optic transmission line. If the outgoing eastward fiber optic transmission line should fail between first and second repeater stations, the electrical switching circuit of the first (west-side) station is switched to establish a loopback circuit between the output of the first O/E signal transducer and the input of the second E/O signal transducer so that the eastward optical signal is looped back in the westward direction at the first station, while the switching circuit of the second repeater station is switched to establish a loopback circuit between the output of the second O/E signal transducer and the input of the first E/O signal transducer so that the westward optical signal is looped back in the eastward direction at the second station.
However, the recent development of a direct light amplification technique using a length of optical fiber doped with a substance of the rare-earth group, such as erbium, and a source of excitation light energy to be injected into the doped fiber, is promising a future fiber optic repeatered transmission system without E/O and O/E signal transducers. It is therefore desirable to incorporate full-optical loopback switching facilities in a repeatered fiber optic transmission system where the direct light amplification technique is employed.